Significant improvements in producing fire and heat resistant structures have been made by the use of intumescent fire retardant compositions involving the chemical reaction of phosphoric acid and a reducing sugar with the possible addition of one or more additives to increase the intumescent action. Compositions of this general type are disclosed in the patent application of Dr. Ralph Matalon, Ser. No. 755,596, filed Dec. 30, 1976 now abandoned. This application relates to resinous intumescent compositions having high fire and heat resistant properties, which were useful in producing fire and heat resistant structures. However, some compositions of this type were found to have the disadvantage that when used in the making of fire resistant panel structures, laminated or otherwise, the problem of moisture sensitivity over a time period or in an especially humid environment limited the use of such structures when exposed to highly humid conditions. It was thus required in some instances where exposure to humid conditions existed that the board structures had to be sealed in some fashion against the humidity or other provisions made to increase usefulness under all types of exposure. The present invention deals with the solution to this problem.